linux-kernel - Re: [RFCv3 PATCH 33/48] sched: Energy-aware wake-up task placement

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Message-ID: <20150324130000.GM23123@twins.programming.kicks-ass.net>
Date:	Tue, 24 Mar 2015 14:00:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Sai Gurrappadi <sgurrappadi@...dia.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFCv3 PATCH 33/48] sched: Energy-aware wake-up task placement

On Mon, Mar 16, 2015 at 02:47:23PM +0000, Morten Rasmussen wrote:
> > Also, this heuristic for determining sg_target is a big little
> > assumption. I don't think it is necessarily correct to assume that this
> > is true for all platforms. This heuristic should be derived from the
> > energy model for the platform instead.
> 
> I have had the same thought, but I ended up making that assumption since
> I holds for the for the few platforms I have data for and it simplified
> the code a lot. I think we can potentially remove this assumption later.

So the thing is; if we know all the possible behavioural modes of our
system we could pre compute which are applicable to the system at hand.

All we need is a semi formal definition of our model and a friendly
mathematician who can do the bifurcation analysis to find the modal
boundaries and their parameters.

I suspect there is a limited number of modes and parameters and if we
implement all we just need to select the right ones, this could save a
lot of runtime computation.

But yes, we don't need to start with that, we can do the brute force
thing for a while.
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